Reader for rfid transponders and corresponding method

ABSTRACT

An RFID reader system comprises one or more transceiver modules ( 10 ) for communicating with a plurality of RFID tags, a control unit ( 8 ) having a reference frequency generator for providing a reference frequency to said one or more transceiver modules ( 10 ), a control bus ( 16 ) connecting the control unit ( 8 ) to said one or more transceiver modules ( 10 ) for communicating with said one or more transceiver modules ( 10 ) and the control unit ( 8 ) having a communications port allowing the control unit to communicate with an application host system. The control unit ( 8 ) provides a common reference frequency to said transceiver modules. The invention also relates to a method of reading tags, comprising the steps of providing a plurality of transceiver modules ( 10 ) with a common reference frequency, and controlling at least one of the transceiver modules ( 10 ) from a control unit ( 8 ) to read one or more of the plurality of tags.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/595,529, filed Mar.26, 2007, which was a national stage filing under 35 U.S.C. 371 ofPCT/GB2004/004505, filed Oct. 25, 2004, which International Applicationwas published by the International Bureau in English on May 12, 2005,which claims priority to GB Application No. 0325026.3, filed Oct. 27,2003, which are hereby incorporated herein in its entirety by reference.

FIELD OF INVENTION

This invention refers to Radio Frequency Identification and moreparticularly to RFID methods and reader systems used to readtransponders or radio frequency identification Tags.

BACKGROUND TO THE INVENTION

Radio Frequency Identification (RFID) systems comprise a Readerotherwise known as an Interrogator or Scanner and a plurality of Tagsalso known as Transponders or Electronic Labels. Systems designed forlow cost applications to identify or track items may use so calledPassive Tags which rely on the transmitted energy field from reader toprovide their operating power and to provide a carrier signal on whichthey can use varying impedance modulation to signal their identity or totransmit their energy for reception by the reader. This form ofmodulation is commonly referred to as backscatter modulation.

Passive Tags may optionally incorporate a battery to provide energy topower the circuits on the Tag but they rely on backscatter modulation tocommunicate their identity. These tags are called Battery assistedPassive Tags. RFID Tagging systems employing these types of tags tocommunicate over ranges of more than 1 metre are designed to operate inthe UHF or microwave bands, typically at frequencies in the region of433 MHz or 860 to 960 MHz or 2.45 GHz. However these are not the onlyfrequencies used for RFID systems.

A variety of communications protocols are used to enable the reading ofmultiple tags present in the operating field of the reader. Theseprotocols are called arbitration protocols, also referred to ascollision arbitration or anti-collision protocols. These protocols mayuse either Aloha hold-off and retry arbitration or tree walking orbinary search protocols to arbitrate populations of tags present. TheInternational Standard ISO/IEC FCD 18000 describes a number of suchsystems in use at different operating frequencies. Tags may be attachedto items examples of which are bottles of detergent or chemicals, itemsof clothing, motor spares, electronic assemblies, cartons, plasticcrates used in transporting perishable foods, pallets, roll cages or anyother item one can imagine.

Tags may also be embedded in consumer products or other items in orderto provide life cycle tracking and management. The items to which tagsare attached affect the radio frequency performance of the tags or thedirection from which tags can be read or the operating range of tags. Inorder to read tags in many situations the reader system must beconfigured as a portal reader in which individual reader antennas aremounted around the portal in order to provide adequate signal levels tothe tags or to permit the reception of the backscatter signal from thetags. The antennas in such a system may be switched or multiplexed toprovide a wider angle of radiation or greater reader volume than ispossible with a single antenna as described in U.S. Pat. No. 6,367,697Entitled “A Reader Arrangement for an Electronic Identification SystemHaving a Plurality of Reader Heads for Energising Transponders”incorporated herein by reference. The antennas in such a system may alsobe combined such that all the antennas are energised simultaneously.

In the multiplexed arrangement it is possible that a number of tags outof the plurality of tags in the reader field may not receive adequateillumination of RF energy from the reader, or that the tags are arrangedsuch that they may be illuminated by the energising signal from thereader but are in a receive null and therefore their backscatter signalfrom the tag is not visible to the receiver in the reader. In thecombined arrangement, all antennas radiate a signal from the reader andtherefore could potentially provide a solid reading field. However dueto reflections and phase differences at different points in the readingvolume resulting from phase changes due to propagation from the multipleantennas, energy nulls may occur in the volume and therefore tags whichare placed in these nulls may never be energised or may never be visibleto the receiver in the reader. It has been observed that it is possiblefor tags to be energised but for the response signal to not be receivedby the receiver in the reader.

Yet another problem with using a single reader with multiplexed multiplereader antennas results from the transmit power and receiver signallosses associated with the loss in the transmitter feedlines used tocouple the reader to the antennas. In a typical 3 metre wide portalsystem, cables can be as long as 12 metres resulting in a power andsignal loss of 6 dB or more depending on the characteristics of thefeeder cable used. It is possible to reduce the loss in the cable byusing coaxial cable having a larger cross sectional area, however thesecables are difficult to install due to their large diameter and weight.They are also expensive and so do not provide a practical solution tothe problem of cable loss.

OBJECT OF THE INVENTION

It is the object of this invention to at least alleviate one or more ofthese disadvantages.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided an RFIDreader system comprising one or more transceiver modules forcommunicating with a plurality of RFID tags, a control unit having areference frequency generator for providing a reference frequency tosaid one or more transceiver modules, a control bus connecting thecontrol unit to said one or more transceiver modules for communicatingwith said one or more transceiver modules and the control unit having acommunications port allowing the control unit to communicate with anapplication host system.

In a preferred embodiment the system comprises a plurality oftransceiver modules, the control unit providing a common referencefrequency to said transceiver modules.

In one embodiment the system comprises a plurality of transceivermodules and the control unit can control at least one of the transceivermodules to read one or more of the plurality of tags. The transceivermodules can be arranged to transmit individually or in groups orsimultaneously. Alternatively, or in addition, the control unit cancontrol each of the transceiver modules to be activated in rotation, orin a random or pseudo-random sequence to read one or more of the tags.

In the system one or more, but not all, of the transceivers may bearranged to transmit an energising signal while one or more of the othertransceivers have their receivers active to serve as diversityreceivers.

In one embodiment of the system, the frequency of at least one or moreof the plurality of transceivers has its reference frequency offset fromthe others thereby providing a low frequency beat note which can beadjusted to fall below the cut off frequency of the detector in thetags.

According to another aspect of the invention, there is provided a methodof reading tags, comprising the steps of providing a plurality oftransceiver modules with a common reference frequency, and controllingat least one of the transceiver modules to read one or more of theplurality of tags. In one embodiment the method includes controlling thetransceivers to transmit individually or in groups or simultaneously.Alternatively, or in addition, the method may include the step ofcontrolling each of the transceiver modules to be activated in rotation,or in random or pseudo-random sequence to read one or more of the tags.

The method may include one or more, but not all, of the transceiverstransmitting an energising signal to the tags while one or more of theother transceivers have their receivers active to serve as diversityreceivers.

In another embodiment the method includes controlling the frequency ofat least one or more of the plurality of transceivers to have itsreference frequency offset from the others thereby providing a lowfrequency beat note which can be adjusted to fall below the cut offfrequency of the detector in the tags.

Other embodiments of the method and system of the invention are definedin the claims.

DESCRIPTION OF THE INVENTION

The invention will be described further, by way of examples, withreference to the accompanying drawings in which:

FIG. 1 is a reader system known in the prior art

FIG. 2 is a block diagram of a reader system according to one embodimentof the invention and

FIG. 3 is a block diagram of a reader transceiver module according to anembodiment of the invention.

A prior art system as illustrated in FIG. 1. It has a reader 4 and aplurality of read heads or antennae (1, 2) activated in atime-multiplexed manner, by multiplexer (6), with a cycle time less thanthe reset times of the transponders or tags associated with the articlesto be counted or identified. The reader is set up as a portal readerarrangement (not specifically shown) and the concept is to try andreduce errors when reading a lot of tags being scanned by the readerarrangement. However, as described above errors can still arise causedby nulls in the volume interrogated within the portal.

FIG. 2 is a reader system and apparatus according to one embodiment ofthe invention. It comprises a control unit (8) separate from one or moreindividual transceiver modules (10) which may or may not be attached toantennas (1, 2 . . . ‘n’.). Each transceiver module (10), shown in moredetail in FIG. 3, incorporates a frequency generator (20,22), atransmitter power amplifier (26), a modulator (24), a receiver (28)having one or more channels, a data decoder (30) for each of thechannels to demodulate or decode the received data stream and acommunications interface (32) used to connect the module (10) to thecontrol unit (8). The transceiver modules (10) are described in moredetail later with reference to FIG. 3.

The control unit (8) has an interface which is used to communicate witha host computer system by means of a serial interface, Ethernet orsimilar and an interface to match that used on the transceiver modulewhich is used to control one or more transceiver modules (10). Thecontrol unit (8) also contains a frequency source which is used as areference frequency for the one or more transceiver modules (10).

The reader may be configured as a single stand-alone reader or may beinstalled as a reader system having multiple antennas.

The transceiver module (10) generates its operating frequency using adirect digital synthesiser (DDS) (20) or phase locked loop synthesiser(PPL) (22) which takes its reference from an externally providedreference frequency. The purpose of the external reference is to permitmultiple transceiver modules (10) to transmit on exactly the samefrequency. However, the transceiver module (10) may use a fractionalsynthesiser to provide a controllable offset from the nominal operatingfrequency.

The control unit (8) provides the intelligence for determining how eachtransceiver (10) behaves and also provides an interface to the hostsystem or application.

The individual transceivers are installed such that they are placed inan arrangement where the radiation patterns of the individual antennas1, 2 . . . n overlap each other to provide continuous coverage of thedesired reader volume.

Each transceiver (10) may be turned on individually by the controller(8) so that each may work as a single stand-alone reader. Eachtransceiver (10) may also be turned on in sequence such that eachantenna (1, 2 . . . n) covers a portion of the reader volume for aportion of the time. All transceivers may be turned on simultaneously sothat the whole reader volume is covered at one time.

The transceivers (10) may be arranged such that only one transceivertransmits an energising signal while one or more of the othertransceivers have their receivers active and so are able to act asdiversity receivers.

The frequency of each of the plurality of transceivers (10) may havetheir reference frequency offset from the others such that there is alow frequency beat note which can be adjusted so that it falls below thecut off frequency of the detector in the tags. By offsetting thefrequency between transceivers (10) which transmit simultaneously, theeffect of the standing nulls may be reduced. In one embodiment thisoffset is achieved by adjusting the phase lock loop synthesiser (22)reference frequency by means of a Direct Digital Synthesiser (20) undercontrol of the control unit. This arrangement is shown in FIG. 3.

Because each transceiver is connected directly to its own antenna, thereare no communications losses associated with antenna feed lines. Allcommunications takes place over the data signal lines.

In one embodiment of the system, a control unit may be interconnected toan array of transceiver modules, each module being connected to its ownantenna by a very short length of coaxial feed line. The frequencyreference signal is fed to each of the transceivers by means of a lowcost flexible coaxial cable (14) because the losses in the cable at thereference frequency is easily compensated for without affecting thefinal operating frequency or the reader transmitter output. The controlunit (8) communicates with all the transceiver modules (10) by way ofthe bi-directional bus (16). The array of transceivers in one embodimentcan be arranged around a typical loading dock door measuringapproximately 3 meters high by 3.5 metres wide. When pallets of items,each pallet may contain as many as 200 individual items tagged with RFIDtags, is moved through the dock door, some items may be hidden from viewwith respect to one or more of the reader transceiver module antennas,however because of the multiple antennas used on the reader, at leastone of the plurality of transceiver module antennas will see each oftags, so that in combination all of the tags will be successfully readby the system.

One problem encountered in existing systems is that a tag may besuccessfully illuminated and powered by the reader's energising field,the return signal from the tag may occur in a null in the reader'sreceiver antenna system. The system described in this inventionovercomes this problem, because the control unit may command all thetransceivers to receive simultaneously but for only one transceiver totransmit. The transceivers which are in the receive only mode act asdiversity receivers, a technique well known in the art of radiocommunications but hitherto a technique which has not been applied toRFID. The transceiver receivers all listen for the tag response signalbut perhaps only two transceivers receive the signal. These transceiversprocess the signal, decode the data stream and pass this stream along tothe control unit (8) over the bi-directional communications bus (16).The control unit (8) will then select the best signal and afterprocessing the data will pass this data along to the application via theapplication host interface.

In another embodiment of the system, two or more transceiver modules(10) may be arranged facing each other on either side of a conveyorbelt. The Control Unit (8) commands both transceiver modules (10) totransmit simultaneously thereby creating a field between the transceiverantennas. As items pass through the field between the antennas, they areilluminated. Because two transceivers are transmitting simultaneouslybut on exactly the same frequency no beat note is generated between thetransmitted carrier frequencies and therefore no deep modulation nullsoccur in the field to interfere with the communication between thereader transceivers and the tags attached to the items moving along theconveyor. This technique has been found to considerably improve thereliability and probability of reading tags moving through a read zone.

In yet another embodiment of the system, two transceiver module antennasare arranged at right angles to each other such that a cubic readingzone is created by the overlapping radiation fields of the two antennas.The transceiver modules (10) are caused to transmit simultaneously thuscreating a reader zone in which tags may be placed in any orientationand still be illuminated and read by at least one of the transceivermodule antennas.

In any of the embodiments, the transceiver modules (10) may be caused toadjust their output power level in order to change the operating orreading range of the module. The Control Unit (8) may also cause theradiated power levels from individual reader modules (10) to be setdifferently in order to adjust the shape and phasing of the reading zoneformed by an array of multiple reader transceiver modules (10).

Each transceiver module 10 may contain, but is not limited to, one ormore of the following components:

-   -   a frequency generating means which may be a direct digital        synthesiser (DDS) (20), or a phase locked loop synthesiser (PLL)        (22) or other 30 method of generating a frequency;    -   A power amplifier (26) which boosts the basic signal to a level        suitable for driving an antenna    -   A modulator (24) to permit commands and data to be modulated on        the transceiver transmitter carrier wave.    -   A transmitter/receiver combining means which may be a        directional coupler or circulator or a directly tapped delay        line or other means    -   A receiver mixer/detector (28) which converts the incoming        signal to a baseband signal, the receiver (28) having separate        “I” and “Q” channels;    -   A decoder (30) which extracts the incoming data stream from the        baseband signal    -   A Bus control section circuit (32) which interfaces the        individual circuits in the module to the communications and        control bus to allow the module to communicate with the separate        control Unit.    -   An optional frequency reference source such as a temperature        compensated oscillator which will allow the transceiver module        to be used as a stand alone reader transceiver.    -   An integral antenna    -   An integral power supply unit.

The reader Control Unit 8 may contain, but is not limited to, thefollowing components

-   -   A communications interface circuit to communicate with one or        more transceiver modules    -   A communications interface circuit to communicate with an 20        application host which may use one of the many industry standard        communications links for example RS232 or RS485 or Ethernet    -   A Reference Frequency source which may be a temperature        compensated oscillator or a simple crystal or a frequency locked        source which may be locked to the 50/60 Hz mains frequency, or a        GPS receiver or an off air source such as television station or        Time and Frequency standard transmission.    -   A power supply unit    -   A communications concentrator to permit a network of similar        control units to be interconnected

In various embodiments the multiple readers or transceiver modules usethe same frequency source and therefore may transmit simultaneouslywithout the emitted transmissions causing interference patterns. Thetransceivers may have their reference frequency offset, as preciselycontrolled offsets can alleviate the standing null problem. In the priorart arrangements, in which different independent frequencies are sent tothe reader heads, if one tried to use frequency offsets, the desiredoffsets would be more difficult to precisely control due, for example,to drift from temperature and aging effects.

In the above embodiments losses may be eliminated and there is providedan arrangement which is lower cost than if individual full readers areused. As described above, advantageously each reader transceiver can actas a diversity receiver and the transceivers can be arranged to transmitindividually or simultaneously.

1. An RFID reader system comprising one or more transceiver modules forcommunicating with a plurality of RFID tags, a control unit having areference frequency generator for providing a reference frequency tosaid one or more transceiver modules, a control bus connecting thecontrol unit to said one or more transceiver modules for communicatingwith said one or more transceiver modules and the control unit having acommunications port allowing the control unit to communicate with anapplication host system.
 2. A system as claimed in claim 1, comprising aplurality of transceiver modules, the control unit providing a commonreference frequency to said transceiver modules.
 3. A system as claimedin claim 2, wherein the control unit can control at least one of thetransceiver modules to read one or more of the plurality of tags.
 4. Asystem as claimed in claim 3, wherein the control unit can control thetransceiver modules to transmit individually or in groups orsimultaneously.
 5. A system as claimed in claim 3, wherein the controlunit can control each of the transceiver modules to be activated inrotation, or in a random or pseudo-random sequence to read one or moreof the tags.
 6. A system as claimed in claim 1, wherein the or eachtransceiver module is provided with a direct digital synthesiser orphase locked loop synthesiser, which takes its reference from thereference frequency transmitted from the control unit, to generate anoperating frequency.
 7. A system as claimed in claim 1, wherein thetransceiver modules are provided with antennas, the antennas beingarranged so their radiation patterns overlap each other therebyproviding continuous coverage of a desired reader volume.
 8. A system asclaimed in claim 1, wherein one or more, but not all, all of thetransceivers are arranged to transmit an energising signal while one ormore of the other transceivers have their receivers active to serve asdiversity receivers.
 9. A system as claimed in claim 1, wherein thefrequency of at least one or more of the plurality of transceivers hasits reference frequency offset from the others thereby providing a lowfrequency beat note which can be adjusted to fall below the cut offfrequency of the detector in the tags.
 10. A system as claimed in claim9, wherein said reference frequency offset is achieved by adjusting thephase lock loop synthesiser reference frequency by means of the DirectDigital Synthesiser under control of the control unit.
 11. A system asclaimed in claim 1, wherein each transceiver module is connected to itsown antenna by a coaxial feed line.
 12. A system as claimed in claim 1,wherein the frequency reference signal from the control unit is fed toeach of the transceivers by means of a flexible coaxial cable.
 13. Asystem as claimed in claim 1, wherein two or more of the transceivermodules are arranged facing each other on either side of a volume to beinterrogated, whereby controlling the modules to transmit simultaneouslyat the same frequency creates no beat note between the transmittedcarrier frequencies so that no deep modulation nulls occur in theenergised field to interfere with the communication between the readertransceivers and the tags.
 14. A system as claimed in claim 1, whereinat least two of the transceiver modules are arranged at right angles toeach other defining a cubic reading zone by overlapping fields from theantennas of the transceivers.
 15. A system as claimed in claim 1,wherein the transceiver modules can adjust their output power level inorder to change the operating or reading range of the module.
 16. Asystem as claimed in claim 15, wherein the control unit can control theradiated power levels from individual reader modules to be setdifferently in order to adjust the shape and phasing of the reading zoneformed by an array of the reader transceiver modules.
 17. A method ofreading tags, comprising the steps of providing a plurality oftransceiver modules with a common reference frequency, and controllingat least one of the transceiver modules from a control unit to read oneor more of the plurality of tags.
 18. A method as claimed in claim 17including controlling the transceivers to transmit individually or ingroups or simultaneously.
 19. A method as claimed in claim 17, includingthe step of controlling each of the transceiver modules to be activatedin rotation, or in random or pseudo-random sequence to read one or moreof the tags.
 20. A method as claimed in claim 17 including one or more,but not all, of the transceivers transmitting an energising signal whileone or more of the other transceivers have their receivers active toserve as diversity receivers.
 21. (canceled)